The incoming fire nerf is something like 4% overall damage, I'm not sure why people are panicking and freaking out thinking they now need to go back arcane in order to minmax.
It's not like you will be put in a "go arcane or get sat" situation in a 1/8 heroic guild this far into the tier. Every heroic fight other than ultraxion and spine favors fire on 25-man, and the utility of cauterize favors fire for ultraxion too. Blackhorn is doable as arcane only if your strat has you specifically tunneling drakes and never having to move. Those simcraft numbers that had arcane not far behind fire were for "patchwerk, no movement", which is approximately zero fights this tier.